1. Field of the Invention
The present invention relates to a disk drive such as a magnetic hard disk drive having a spindle motor with a stator and a rotor that is rotatable about the stator desirably in a forward-spin direction and undesirably in a reverse-spin direction. More particularly, the present invention relates to controlling the angular velocity of the rotor during spin-down and subsequent spin-up procedures.
2. Description of the Prior Art
A huge market exists for hard disk drives for mass-market host computer systems such as servers, desktop computers, and laptop computers. To be competitive in this market, a hard disk drive must be relatively inexpensive, and must accordingly embody a design that is adapted for low-cost mass production. In addition, it must provide substantial capacity, rapid access to data, and reliable performance. Numerous manufacturers compete in this huge market and collectively conduct substantial research and development to design and develop cost effective technologies including structure and methods for rapidly performing spin-down and subsequent spin-up procedures.
Each of numerous disk-drive companies manufacture and sell hard disk drives that have similarities in basic design pursuant to which the disk drive includes a head disk assembly ("HDA") and a printed circuit board assembly ("PCBA") connected to the HDA. The RDA includes at least one disk and a spindle motor. The PCBA includes circuitry for controlling the operation of the spindle motor.
As for the basic construction of the spindle motor, it has stationary elements, collectively referred to as a stator, and it has rotatable elements, collectively referred to as a rotor. The rotor includes a hub that supports each disk. The spindle motor includes a bearing arrangement such as sets of ball bearings or a journal bearing so that the rotor is rotatable about the stator. The stator includes electromechanical stator structure; this structure includes a stator core having a plurality of core members that radiate away from the center of the core to define stator poles, and a set of stator wires that are wound around the stator poles and interconnected in a predetermined configuration such as a "Y" configuration of windings. The rotor includes a set of permanent magnets that are arranged to define rotor poles.
As for the basic operation of the spindle motor in a disk drive, the rotation of the rotor causes each disk to spin. It would be desirable for the rotation always to be in a forward-spin direction; undesirably, the rotor can rotate in a reverse-spin direction. To accelerate the rotor to a normal operating spin rate, and then to maintain the spin rate substantially constant, applied torque must be generated first to overcome stiction and, then to oppose dynamic frictional forces. To generate the applied torque, current is caused to flow through one or more of the windings to generate an electromagnetic field referred to herein as a stator magnetic field. The magnitude of the stator magnetic field is variable; it depends upon the magnitude of current flow. The instantaneous angular direction of the stator magnetic field is also variable. The permanent magnets in the rotor produce a rotor magnetic field. The stator magnetic field and the rotor magnetic field interact. When the two fields are exactly intersect, the force on the rotor is maximized and the applied torque is zero; otherwise, applied torque is generated. The applied torque has a magnitude that varies as a periodic function of the rotor angle.
As for the basic construction of the circuitry for controlling the operation of the spindle motor, it includes switching elements and a state machine for controlling the operation of the switching elements. In a disk drive, the terminals of the stator windings are electrically connected to the output terminals of the switching elements. Typically, the state machine and the switching elements are part of a single integrated circuit chip referred to as a spindle motor driver. The state machine that can be implemented by dedicated circuitry within the spindle motor driver chip, by a programmed microprocessor, or otherwise. The state machine includes a register set and combinatorial logic interconnected with the register set. The register set defines one state at a time out of a plurality of register states. The state machine has an input for receiving a set of digital input signals and has an output for producing a set of output digital signals. At each instant, the values of the set of output digital signals depend upon the present register state and the values of the set of digital input signals. Also, the next register state to which the state machine will step depends upon the present register state and the values of the set of digital input signals. The state machine defines a set of transitions between the states in accordance with a state diagram.
As for the basic circuit operation involving the control of operation of the spindle motor, the circuitry performs a process referred to as "commutation." The commutation process can cause the angular direction of the stator magnetic field to be rotated in a way that causes the rotor to "follow" it. The term "commutation" is used in several senses in this art. As for its dictionary meaning with reference to a DC motor and an external circuit for supplying current to it, the dictionary meaning is "a reversal or transference" between a winding and the external circuit. In this art, the term "commutation" is also used in reference to operating conditions caused by and corresponding to a state defined by the state machine in the external circuitry. When the term commutation is used in the sense of a reversal or transference, it refers to a change caused by a transition in the state diagram. That is, the operating condition of the switching elements change upon each transition in the state diagram; likewise there is a corresponding change in the instantaneous angular direction of the stator magnetic field upon each transition in the state diagram. The term is also used in the phrase "commutation phase" in reference to the operating condition corresponding to a state defined by the state machine.
During an interval of time, the set of output digital signals produced by the state machine can define a predetermined sequence out of a plurality of predetermined sequences. Such a predetermined sequence can be cyclical. Such a cyclical sequence is referred to herein as a commutation sequence. For example, throughout normal operation while the rotor is spinning at its normal operating spin rate, the state machine steps through a commutation sequence to cause the instantaneous stator magnetic field to rotate in direction consistently with the rotation of the rotor magnetic field.
In a spindle motor driver chip, the switching elements are typically implemented by tristate CMOS switching elements. These switching elements are controlled by the combinatorial logic in accordance with the register state of the state machine. These switching elements are connected to the terminals of the stator windings and switch on and off to control the flow of current through the windings. Three items discussed above can be related on a one-to-one correspondence basis in a truth table; these being: 1) the state of the register set in the state machine; 2) the operating condition of the switching elements; and 3) the instantaneous angular direction of the stator magnetic field.
The terms "unipolar," "bipolar," and "tripolar" are used in this art in reference to modes of operation with respect to current flow through the windings of a three-phase motor. See an article authored by Raffi Codilian and Don Stupeck titled "A MULTI MODE SPINDLE SELECTION WITHIN A DISK DRIVE SYSTEM," published in the Incremental Motion Control Systems Symposium in 1995.
In unipolar mode, current flows through a single winding between one of the winding terminals and the centertap, while no current flows through either of the other windings. In bipolar mode, current flows through two of the windings between two of the winding terminals, while no current flows through the other winding. In tripolar mode, current flows through all three of the windings.
With reference to the graphs shown in FIGS. 1A, 1B, and 1C, in each case, the abscissa represents the rotor angle (the angular position in electrical degrees relative to the stator), and the ordinate represents applied torque on the rotor resulting from the interaction of (1) the stationary magnetic field produced electromagnetically for the present stator condition pursuant to the register state corresponding to the depicted torque curve, and (2) the rotating magnetic field produced by the permanent magnets in the rotor. For each register state of the state machine, there are two rotor angles where the two fields exactly intersect. One of these is called a stable equilibrium position and the other is called an unstable equilibrium position.
In FIG. 1A, a representative torque curve T1 has an unstable equilibrium position (UEP) at the origin (0 electrical degrees), a stable equilibrium position (SEP) at 180 electrical degrees, and an unstable equilibrium position (UEP) at 360 electrical degrees. The torque curve T1 defines a peaked forward-direction waveform (PFW) throughout the angular interval between 0 and 180 electrical degrees. Any rotor angle in the range between 0 and 180 electrical degrees can be said to be "within the range of the peaked forward-direction waveform" for register state corresponding to the torque curve T1. The torque curve T1 defines a peaked reverse-direction waveform (PRW) throughout the angular interval between 180 electrical degrees and 360 electrical degrees.
Consider a circumstance in which the rotor is stationary and positioned at the stable equilibrium position (SEP) at 180 electrical degrees. If the rotor were to be disturbed for any reason such that it rotates in the forward-spin direction, the applied torque on the rotor would change from zero to a negative value that would urge the rotor to return to the stable equilibrium position (SEP) at 180 electrical degrees. Similarly, if the rotor were to be disturbed for any reason such that it moves in the reverse-spin direction, the applied torque on the rotor would change from zero to a positive value that would urge the rotor to return to the stable equilibrium position (SEP) at 180 electrical degrees.
Consider a circumstance in which the rotor is stationary and positioned at the unstable equilibrium UEP at 0 electrical degrees. If the rotor were to be disturbed for any reason such that it rotates in the forward-spin direction, the applied torque on the rotor would change from zero to a positive value that would urge the rotor to continue rotating in the forward-spin direction. Similarly, if the rotor were to be disturbed for any reason such that it rotates in the reverse-spin direction, the applied torque on the rotor would change from zero to a negative value that would urge the rotor to continue rotating in the reverse-spin direction.
In FIG. 1B, a representative torque curve T4 has a stable equilibrium position (SEP) at the origin, an unstable equilibrium position (UEP) at 180 electrical degrees, and a stable equilibrium position (SEP) at 360 electrical degrees. The torque curve T4 defines a peaked reverse-direction waveform (PRW) throughout the angular interval between the origin and 180 electrical degrees. The torque curve T4 defines a peaked forward-direction waveform (PFW) throughout the angular interval between 180 electrical degrees and 360 electrical degrees.
The torque curves of FIGS. 1A and 1B differ in phase by 180.degree.. Although the same windings have the same magnitude of current flowing for the torque curves of FIGS. 1A and. 1B, the current directions are opposite.
In FIG. 1C. six separate torque curves (T1) through (T6) are superimposed on the same graph.
Consider now the question of how to start to start to perform a spin-up procedure to cause the rotor of the spindle motor to accelerate to its normal operating spin rate from the position at which it last stopped. Consider a circumstance in which, at the time a spin-up command is received, the rotor angle is at 270 electrical degrees (the starting rotor angle). If the spin-up procedure were to start with torque curve T4, desirable results would follow in that the applied torque would be in the forward-spin direction. In this case, the starting rotor angle is within the range of the peaked forward-direction waveform for torque curve T4. On the other hand, if the spin-up procedure were to start with torque curve T1, undesirable results would follow in that the applied torque would be in the reverse-spin direction. In this case, the starting rotor angle is outside the range of the peaked forward-direction waveform for torque curve T1.
An abundance of tutorial material exists about disk drive technology relevant to the construction and operation of spindle motors including spin-down and subsequent spin-up procedures. Reference is made to the following representative patents:
U.S. Pat. No. 5,397,972, titled "START-UP PROCEDURE FOR A BRUSHLESS, SENSORLESS MOTOR" (the '972 patent);
U.S. Pat. No. 5,530,326 titled "BRUSHLESS DC SPINDLE MOTOR STARTUP CONTROL" (the '326 patent);
U.S. Pat. No. 5,623,379 titled "METHOD OF CONTROLLING A START-UP OF A MOTOR USED FOR A DISK APPARATUS" (the '379 patent);
U.S. Pat. No. 5,471,353 titled "DISK DRIVE EMPLOYING MULTI-MODE SPINDLE DRIVE SYSTEM" (the '353 patent);
U.S. Pat. No. 5,650,886 titled "DISK DRIVE SPINDLE MOTOR STARTUP USING AN ADDITIONAL MOTOR WINDING UPON STARTUP FAILURE;" and
U.S. Pat. No. 5,223,771 titled "POLYPHASE BRUSHLESS DC MOTOR CONTROL" (the '771 patent).
The '972 patent shows torque curves in accordance with a presentation convention in which forward-spin direction extends from right to left, whereas FIGS. 1A through 1C show torque curves in accordance with a different presentation convention in which forward-spin direction extends from left to right. There is no substantive difference between these drawings. The '972 patent is directed to an approach intended to provide a fast start-up procedure which will prevent backward rotation irrespective of the starting angular position of the rotor.
The '326 patent is directed to an approach intended to detect starting rotor angular position.
The '379 patent is directed to an approach involving spindown to a stop, followed by a procedure normally used as part of response to a spin-up command, followed by an indefinite time period, followed by a spin-up procedure.
The '771 patent shows in its FIG. 4 a flow chart of actions typically taken during a spin-up procedure. One action is referred to as "COG MOTOR" and is described in the text as moving "the rotor of the motor to a specific phase position." This action takes time and it is desirable to reduce the amount of time taken to carry out a spin-up procedure. FIG. 1D hereof presents a timing diagram relating to the overall time for effecting a spin-up procedure according to the prior art taught in the '771 patent. One part of the overall time is an interval 110 during which the COG MOTOR action takes place. Next is an interval 112 during which a blind spin action takes place. Next is an interval 114 during which Back ElectroMotiveForce ("BEMF") sensing is employed in a servo process for controlling the acceleration of the rotor up to the desired spin rate.
There is a need for an improved method of controlling the spinning down and subsequent spinning up of the spindle motor in a disk drive. Preferably, the improved method reduces the time required for spinning up the spindle motor, and reduces head/disk wear of the kind that results from spinning the disk in the reverse-spin direction while the head is bearing against the disk.